1. Field of the Invention
This invention pertains generally to systems and methods for optical data storage. More specifically, the present invention relates to an optical data storage system utilizing multi-layered optical storage media comprising a single dedicated servo layer and a plurality of data layers, with each data layer providing an areal density comparable to that of conventional single data layer media. Separate servo and read-write laser beams operate at dual foci within the media, and separate dedicated and embedded servo systems, associated with the servo and read-write beams, provide focus and tracking error correction.
2. The Background Art
Optical information storage technologies have provided increasing storage densities over the years. The demand for greater optical storage densities has been persistent, and various approaches to increased optical storage densities have been considered. Conventional far-field techniques for reading and writing optical media utilize a laser beam focused onto the data plane of an optical medium by an objective lens. For a laser beam of wavelength xcex and an objective lens with a numerical aperture NA, a read/write spot size of approximately xcex/2NA is obtained. Conventional techniques currently allow single data layer optical media having storage capacities of between about 2.6 GB and about 4.77 GB in currently used 120 mm DVD optical disks.
Diffraction limitations imposed on the read/write spot size by the light wavelength and numerical aperture (NA) of the focusing optics provide limitations on optical media storage capacity. Increasing the NA of the focusing objective lens to greater than approximately 0.6 results in rapid increases sensitivity to tolerances and results in beam aberrations. Use of shorter wavelength semiconductor lasers will allow increased storage densities in the future, but shorter wavelength laser devices have so far tended to have limited output powers, limited operational temperature ranges, and are subject to materials limitations which have so far resulted in poor reliability and relatively rapid deterioration. The shorter wavelength lasers also reduce wavelength tolerance.
One approach to increased optical storage densities has been through development of near-field optical data storage techniques, which require the use of radiation source apertures and distances on the order of generally less than the wavelength xcex of the radiation source to allow high storage densities. One near-field technique involves use of a solid immersion lens (SIL) positioned between the objective and the optical medium to provide an increase in NA which is proportional to the refractive index of the SIL material. The use of a SIL, however, is subject to the refractive index limitation of SIL materials. Still another near-field method utilizes tapered optical fibers with metallized sides. While tapered fibers have provided small spot sizes, they are severely limited in output power, and are subject to catastrophic breakdown at the emission aperture. Perhaps the most important drawback to near-field technologies, however, is imposed by the necessary close spacing of the optical medium and light aperture, which requires the use of a flying head. The flying optical head, using a SIL or tapered fiber, adds cost and complexity to storage systems, and the flying height of the head can result in head/disk contact and poor reliability. These problems do not occur with far-field systems.
Another approach to increased optical data storage density has been through use of multiple data layers on a single substrate. This is most easily achieved by placing a single data storage layer on each side of a substrate to provide a dual sided optical medium having effectively twice the storage density of a single-sided optical medium. Dual sided media, however, inconveniently require that the optical disk be xe2x80x9cflippedxe2x80x9d in order to read the opposite side. Dual optical heads can be used with the media to avoid flipping the medium, but result in substantially higher drive costs.
A more attractive multi-layer optical medium would utilize multiple data layers which are addressable from a single side of the optical medium. However, the reading and writing of an underlying data layer through an overlying outer data layer or layers on a single sided medium introduces numerous complexities. Reduced optical transmission to an underlying data layer through overlying layers, potential cross-talk between adjacent data layers, low signal-to-noise rations, and spherical aberration introduced by the thickness of multiple layers, have presented serious limitations to multi-layered optical media. Heretofore, the only commercially useful single side, multi-layer optical medium has involved dual stamped substrates which are sandwiched together with a spacing of about 60 microns, with substantial de-rating (by a factor of two or more) of the inner and/or outer substrate being required to avoid spherical aberration. The de-rating of the inner data layer results in only a limited increase in areal storage density compared to single side, single layer media. Further, the optical transmission and spherical aberration considerations noted above have limited such media to only two data layers.
There is accordingly a need for an optical data storage system and method that utilizes multiple data layers on a single substrate which allows the same storage capacity on each data layer as is available in single data layer optical media, which provides more than two data layers addressable from a single side of the medium, which provides good optical transmission to underlying data layers through outerlying data layers, which avoids cross-talk between adjacent data layers, and which does not require spherical aberration correction. The present invention satisfies these needs, as well as others, and generally overcomes deficiencies found in currently available optical data storage systems.
The present invention is an optical information storage system using optical storage media including multiple data layers or stacks wherein each of the multiple data stacks has a storage density comparable to a conventional single layer optical disk. The optical media of the invention thus provide a high areal storage density.
In general terms, the invention comprises an optical medium having a single dedicated servo reference layer and multiple data stacks which each contain an embedded servo format, a servo laser beam positioned to maintain a first focus point on the dedicated servo reference layer, a read-write laser beam positioned to maintain a second focus point on one of the data stacks, a first, dedicated servo system which provides focus and tracking error correction according to error signals generated from the dedicated servo layer, and a second, embedded servo system which provides focus and tracking error correction according to error signals generated from the data stacks. The dedicated servo layer, in different embodiments of the invention, may be positioned either below or above the data stacks in the optical medium, or interposed between data stacks. The data stacks may comprise discrete physical data layers or xe2x80x9cvirtualxe2x80x9d data layers defined by a format hologram. The servo and read-write lasers may differ in wavelength and/or polarization.
By way of example, and not of limitation, in one presently preferred embodiment the optical medium comprises a dedicated servo layer together with a lower or innermost data stack proximate to the servo layer, and at least one overlying or outer data stack positioned above or outside the innermost data stack. More preferably the medium comprises first, second, third and fourth data stacks positioned above the dedicated servo layer, with the first data stack being outermost, and the fourth data stack being innermost and located adjacent the dedicated servo layer. Each data stack comprises a layer of read-write material surrounded by or positioned between at least two dielectric layers.
The read-write material layer in each data stack may comprise any material which, under write conditions by the read-write laser, can undergo an optically detectable change. The read-write material layer thus may comprise any conventional WORM (write-once-read many), ROM (read-only-memory) or reversible read-write material, including ablative, dye-polymer, photopolymer, ferroelectric, magneto-optic and other materials commonly used in optical storage media. In the presently preferred embodiments, the read-write material layer comprises a phase change material such as a GeSbTe (Germanium Antimony Tellurium or xe2x80x9cGSTxe2x80x9d) alloy which, under sufficiently high laser irradiation during write conditions, undergoes an optically detectable phase change between a crystalline or polycrystalline phase and an amorphous phase.
The dielectric layers of each data stack may comprise any dielectric material having suitable properties to act as thermal and mechanical barriers for the interposed read-write material layer, and having suitable refractive indices as discussed below. In one preferred embodiment, the dielectric layers comprise ZnS, SiO2, and/or ZnS/SiO2.
The data stacks are separated from each other by a spacer layer. The spacer layers may comprise any interlayer material with suitable optical properties, and preferably comprise an optical quality polymer material. The spacer layer may be formed by spin coating a UV-curable resin followed by curing, or by application of a transfer film or contact tape. A spacer layer is also preferably included between the innermost data stack and the dedicated servo layer. The spacer layers may also comprise a vapor-deposited parylene material.
The read-write laser and servo laser preferably operate at different wavelengths. A dye or dye-doped polymer layer, which is highly absorbing to the read-write laser wavelength and highly transparent to the servo laser wavelength, is preferably located between the dedicated servo layer and the innermost or bottom data stack, so that light from the read-write laser does not reach the dedicated servo layer. In one presently preferred embodiment, data reading and writing are carried out using a red laser at about 660 nm, and servo functions associated with the dedicated servo layer are carried out with a near infrared laser at about 780 nm. Various dyes are suitable for absorption of the red laser light and transmission of the near infrared laser light, including merocyanine, hemicyanine, phthalocyanine, spiropyran and other dyes.
The dedicated servo layer preferably comprises a stamped or embossed servo grating or pattern on a plastic or like substrate. The embossed servo pattern preferably comprises a plurality of grooves and lands which define a servo surface with a plurality of tracks. The embossed servo pattern in the dedicated servo layer preferably includes a reflective coating such as gold or a like reflective metal layer.
The thickness of the read-write material layer in each of the data stacks is carefully controlled or determined according to optical absorption and transmission considerations for both the servo and read-write lasers. Thus, the outermost data stacks will generally utilize a thinner layer of read-write material to improve optical transmission to the inner data stacks. The innermost data stack will generally utilize a thicker layer of read-write material to make up for reduction in optical transmission of the read-write beam through the outer data stacks. In other words, the relative thickness of the phase change material layers for the inner and outer data stacks are designed to equalize the absorption for each data stack while permitting sufficient transmission to underlying data stacks.
The range of thickness available for the read-write material layers in the data stacks may be limited according to transient heat transfer considerations. In embodiments using GST phase change material in the read-write material layer, the thickness of the phase change material layer has a lower limit below which the amorphous-to-crystalline phase change occurs too slowly to permit useful initialization rates, and an upper limit above which the crystalline-to-amorphous phase is difficult to induce. The particular thickness range of the phase change material layer will vary according to the particular phase change material used with the invention. More flexibility in the thickness of the phase change material layers can be achieved through use of thermal quenching metal layer in association with the phase change material layer. However, the use of thermal quenching layers are generally less preferred, as increased laser power is required for writing, and optical transmission is generally reduced by the quenching layer.
The thickness and number of dielectric layers associated with each of the data stacks is also carefully controlled according to considerations involving optical absorption and coherent inter-stack interaction. As noted above, a lower absorption at the read-write material layer is generally desirable in the outermost data stacks, in order to improve overall transmission to the innermost data stack, and a higher absorption at the read-write material layer of the innermost data stack is desirable to make up for attenuation of the read-write beam by the outer data stacks. In this regard, the dielectric layers adjacent to the read-write material layers are structured and configured to act as thin film interference filters in association with the adjacent read-write material layers. In the outermost data stacks, the refractive index and thickness of the dielectric layers is tailored to minimize the electric field strength and corresponding absorption of the read-write laser at the read-write material layer, while at the innermost data stack the refractive index and thickness of the dielectric layers are designed to increase or maximize the electric field strength and absorption of the read-write laser at the read-write material layer. The careful design of these interference filters allows these properties to be optimized for the optical media of the invention.
In one preferred embodiment, a single pair of dielectric layers of selected refractive index and thickness are used in each stack, with the read-write material layer interposed between the pair of dielectric layers. The dielectric layer thickness and refractive index are selected to minimize the electric-field strength in the read-write material layer of the outer data stacks, and to maximize the electric-field strength in the read-write material layer of the inner data stack, as noted above. The dielectric layers may, in one embodiment, each approximate quarter wave (xcex/4) layers, such that the data stack approximates a half wave (xcex/2) stack with respect to the wavelength of the read-write laser. This arrangement reduces electric field strength and minimizes absorption in the outer data stacks, and provides for matching of optical admittance to minimize reflection in the outer data stacks.
In other embodiments of the invention, a larger number of dielectric layers may be used in each data stack, with the dielectric layers above the read-write material layer preferably configured to approximate a high-low (HL) quarter wave stack, and with the dielectric layers below the read-write material layer preferably configured to approximate a low-high (LH) quarter wave stack. In the outer data stacks, the HL stack reduces the electric field of the read-write beam at the read-write material layer, while the LH stack matches the optical admittance to maximize optical transmission. The larger number of dielectric layers may, in some embodiments, increase stack thickness and decrease the wavelength tolerance of the optical medium.
In additional embodiments of the invention, a reflective layer may be associated with the outermost dielectric layer of the innermost data stack. Preferably, a gold (Au) film is used as a reflective layer. In other embodiments, the reflective layer may comprise a dielectric stack, or another relatively low energy loss metal film such as silver (Ag) or alloy thereof.
The spacing between the multiple data stacks of the optical medium is preferably controlled by spacer layers positioned between each data stack. The thickness and material of the spacer layers, and thus the spacing between the data stacks, is carefully controlled to minimize coherent interaction between adjacent data stacks. The read-write beam, when focused in a data stack, will have axial lobes of relatively high intensity which can result in cross-talk or interference with an adjacent data stack if the adjacent data stack is too close. The location of the axial lobes are dependent on the numerical aperture of the focusing objective and the wavelength of the read-write beam. The axial lobe location is determined from physical optics considerations. Generally, the distance between the adjacent data stacks must be large enough such that the axial lobes resulting from focus of the read-write beam on one data stack do not affect or otherwise significantly interact with adjacent data stacks. The spacing between adjacent data stacks also is preferably small enough such that spherical aberration correction is unnecessary. The presently preferred spacer layers comprise UV-curable resin which is spin-coated to a desired thickness, as noted above.
In the dedicated servo layer, servo information is provided which includes, inter alia, a plurality of focus and tracking servo bursts positioned in servo burst sectors, with individual servo bursts in each sector positioned in a quadrature arrangement. Preferably, each focus and tracking servo burst sector includes a first set of servo bursts positioned at zero degrees according to their respective tracks, a second set of servo burst positioned at one hundred and eighty degrees, a third set of servo bursts positioned at ninety degrees, and a fourth set of servo bursts positioned at two hundred and seventy degrees. This quadrature servo pattern allows the servo beam to be servoed at any radial position on the embossed servo pattern (and thus the optical medium) without the need for a radial offset mechanism. Preferably, each data stack includes embedded servo information in the form of servo bursts, which may also be positioned in a quadrature arrangement, for tracking servo functions associated with the read-write beam. The servo bursts in the dedicated servo layer and the embedded servo bursts may be configured as either AC or DC bursts. The use of AC servo bursts offers the advantage of use of an AC coupler, but may tend to require more surface area of the optical medium.
Dual or separate foci for the servo laser beam and read-write laser beam are separately utilized for simultaneously addressing the dedicated servo layer and the data stacks, with the dual foci formed at different depths in the optical medium. Static control of the dual foci is provided by a first movable lens or objective element having high dispersion for the different servo and read-write laser wavelengths via chromatic aberration or other dispersive property, and/or use of wavefront curvature in another objective element used in association the first movable lens. Both the servo beam and read-write beam are focused on the optical medium by the first movable objective element. Dynamic focus control is provided by axial translation of the first movable objective element, as well as axial translation of one or more additional movable lenses, according to instructions from the dedicated and embedded servo systems.
The first or dedicated servo system of the invention preferably utilizes multiple detectors for detection of the servo laser beam reflected from the dedicated servo layer in the optical medium. A dedicated servo control processor receives focus and tracking error signals from the detectors according to the embossed quadrature pattern servo bursts on the dedicated servo layer, and generates responsive focus and tracking error correction signals. One or more movable lens elements, which focus the read-write and/or servo beams into the optical medium, are axially and laterally translated, according to the focus and tracking error correction signals from the control processor, to provide focus and tracking error correction. Preferably, tracking error correction by the dedicated servo system is carried out by lateral positioning of the first movable lens through which both the servo and read-write beams pass. Focus error correction is preferably carried out by the dedicated servo system by axial positioning of a second movable lens, through which only the servo beam passes.
The second or embedded servo system provides for focus and tracking error correction according to signals derived from the read write beam reflected off one of the data stacks. The read-write laser beam, after reflection from a data stack, is split along three paths for recovery of data and generation of focus and tracking error signals. The three paths are focused respectively through three pinholes to three separate detectors. Tracking error signals according to embedded servo bursts, as well as data, are recovered from one detector associated with a pinhole positioned in the confocal plane. The tracking error signals are directed to an embedded servo control system which generates responsive tracking error correction instructions to offset tracking control of the first movable objective element by the dedicated servo system. The two remaining pinholes are offset from the confocal planes of their respective paths to allow generation of focus error signals which are detected by the two other detectors. The focus error signals are derived from the difference of the signals from the detectors positioned behind the two pinholes, and embedded servo focus bursts are not required in the data stack, thereby providing more space for data storage. The focus error signals are directed to the embedded servo control system which generates responsive focus error correction instructions which are used for axial positioning of one or more objective element. Preferably, focus error correction by the embedded servo control system is carried out by axial positioning of the first movable objective element.
In one preferred embodiment, an integrated holographic optical element is used to split the reflected, focused read-write beam into three paths by diffraction. Preferably, the holographic optical element is a binary optic lens which is structured and configured to generate minus first order (xe2x88x921st), a zeroth order (0th), and plus first order (+1st) diffractions from the read-write beam, with data and tracking error signals recovered from the 0th order diffraction, and focus error signals derived from the xe2x88x921st order and +1st order diffractions. Higher order diffractions are also produced from the holographic optical element and may also be used, but are generally less preferred.
The three pinholes associated with the three diffracted paths preferably comprise three co-axial pinholes aligned in an array configured to capture the xe2x88x921st, 0th and +1st order diffractions from the holographic optical element. The holographic optical element and pinhole array are configured and positioned such that the central pinhole of the array lies in the confocal plane of the 0th order diffraction, and the outer two pinholes are axially offset with respect to the confocal planes of the xe2x88x921st and +1st order diffractions. The holographic optical element, pinhole array, and associated beam detectors and other optical elements preferably are kinematically mounted using multiple precision milled mounting elements. The mounting elements, optical elements and adhesive used to join the optical elements to the mounting elements are matched in coefficient of thermal expansion (CTE) to minimize alignment distortion due to temperature fluctuation.
The pinhole array may be created in-situ by placing a mirror in the object plane of the optical system, positioning a photosensitive absorbing layer at the confocal image plane, and then operating the read-write laser at high output power to burn, photo-bleach, or otherwise open or create the pinholes in the photosensitive layer. The laser output power during pinhole burning must be great enough so that the xe2x88x921st order, 0th order and +1st order diffractions generate their respective pinholes. The mirror in the object plane preferably has suitably high thermal conductivity to avoid damage to the mirror at this high output power. In-situ pinhole burning in this manner eliminates the need for high precision alignment of the pinholes, as would be necessary if the pinholes were fabricated separately. The absorbing film may comprise a thin dye layer or a thin metal layer such as Tellurium coated on a glass, polycarbonate, or other substrate. The size of the pinholes can be controlled through adjustment of exposure time and laser power. The preferred pinhole size is preferably about the same size as the beam spot size at the confocal image plane or smaller.
Further advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing the preferred embodiment of the invention without placing limitations thereon.